1. Field of the Invention
This invention relates to the production of windings of glass thread wound at constant speed, and in the form of tapered windings.
2. Description of the Related Art
Windings of thread are a common means of temporarily storing the thread. The threads can be in different forms: a single thread comprising one twist, twisted threads, etc. They are ultimately fed to textile machines operating at high speed. The thread must be able to be easily unwound while avoiding any friction that could cause a break. In this regard, tapered windings offer a particular advantage compared to other types of windings. In such a winding, the thread, carried along the axis of the winding in the direction of its smallest diameter, moves immediately away from the lateral edge of the spool as soon as a turn pulls away from it. The risk of a turn being held back by an adjacent turn or of the thread rubbing on the lateral edge of the winding is thus very small.
A large number of solutions have been proposed to achieve such windings. They can be produced by winding the thread with a thread guide that moves in a to-and-fro or reciprocal movement parallel to the axis of a tapered support, the latter being rotated by driving rollers initially applied thereto and then applied on the deposited layers of thread.
Some of the known solutions have the object of maintaining the winding speed of the thread approximately constant, despite the continuous variation in diameter of the support on which it is wound. For this it is necessary to make the rotation speed of the support vary so that the thread always encounters a surface whose peripheral speed is approximately constant. Since the support is put in rotation by driving rollers, the maintenance of peripheral speed can be achieved by the alternation of rapid braking and acceleration of said rollers.
A series of solutions using such a process are described in application EP-A-0 343 540, which itself proposes a particular solution.
The difficulties that must be overcome in using such a process are numerous and far from insignificant. Among the latter are the acceleration and braking of the driving rollers that must be perfectly controlled to avoid slipping between the two surfaces in contact. This risk limits the speed at which the thread can be wound; the above-cited document gives an example according to which the speed of the thread is 140 m/min.
Another difficulty is to prevent the pressure that the driving rollers unavoidably exert on the spool from destroying the thread. This is all the more difficult to avoid when the thread is sensitive to friction by its very nature; this is particularly the case with glass threads.
It must also be noted that it is not possible to wind a thread with driving rollers on a support provided with a lateral flange or edge at one of its ends.
Other solutions make it possible to avoid using driving rollers, such as for example the patent U.S. Pat. No. 3,218,004.
This patent describes a process that makes it possible to produce a tapered winding on a cylindrical support provided with a straight lateral flange or edge at each of its ends. This result is achieved by a concomitant variation in the speed of the thread guide and in the rotation speed of the spindle carrying the support. The variation in the speed of the spindle is caused by the variation of the driving torque, itself caused by the variation in the tension of the thread during its winding.
This process has a certain inertia and is applicable only to threads whose mechanical behavior makes it possible to absorb variations in tension, such as wires, but it is not applicable to threads that do not have this ability to absorb such variations in tension, such as glass threads.